1. Field of the Invention
The present invention generally relates to a malfunction detecting apparatus, and more particularly to an apparatus for detecting a malfunction in an evaporated fuel purge system in which evaporated fuel from a fuel tank is stored in a canister, and fuel vapor is fed from the canister into an intake passage of an internal combustion engine.
2. Description of the Related Art
Conventionally, in an evaporated fuel purge system, fuel evaporated in a fuel tank is fed into a canister through a vapor passage, and the fuel is stored in an adsorbent in the canister. The stored fuel is fed into an intake passage of an internal combustion engine through a purge passage. In this purge passage, a purge control valve is provided to control a flow of such fuel vapor being fed into the intake passage. In order to detect a malfunction in this evaporated fuel purge system, a malfunction detecting apparatus has been proposed by the same inventor as that of of the present invention. Such an apparatus is disclosed in the co-pending U.S. patent application Ser. No. 774,589 filed on Oct. 10, 1990. The disclosure thereof is hereby incorporated in the present specification.
In the above mentioned apparatus, a diagnosis control valve is mounted in an air inlet passage connecting the canister to the atmosphere, and a pressure sensor is mounted in this air inlet passage between the canister and the diagnosis control valve. A diagnostic process is carried out by the apparatus from a time when both the purge control valve and the diagnosis control valve are closed to a time when only the purge control valve is opened (the diagnosis control valve is still closed), so that it is detected whether or not a malfunction has occurred within the evaporated fuel purge system, in response to a difference between sensed pressures in the air inlet passage detected by the pressure sensor at the time when the purge control valve is closed and at the time when the same is opened. During the diagnostic process, the vapor passage and the fuel tank inside are subjected to vacuum pressure through the intake passage.
However, the evaporation of fuel in the fuel tank is highly active when the temperature of the fuel is high. If the vapor passage and the purge passage are closed when the fuel in the fuel tank is in such a condition, the pressures of the evaporated fuel purge system vary irregularly, so that a malfunction may incorrectly be detected due to such fluctuations in the sensed pressures. Also, the above mentioned apparatus can detect only a considerable change which has occurred in the sensed pressures, for example, a change due to separation of a connecting pipe in the vapor passage or the like. However, it is difficult to correctly detect a malfunction in response to a relatively small increase or a slight change in the pressures sensed by the pressure sensor. More particularly, there is a problem in that the above mentioned apparatus cannot correctly differentiate between a slight increase in the sensed pressures due to a small fuel leakage and a slight increase in the sensed pressures due to active fuel evaporation when the fuel in the fuel tank is at a high temperature, thus causing erroneous detection of a malfunction in the evaporated fuel purge system.